Sometimes the main function of a will is to give your executor the authority to collect your assets and cause them to be distributed to a trust that you have set up to receive them. This type of will (sometimes called a "pour-over" will) is often used when a trust is set up for estate tax savings, or to preserve family privacy. Remember that a will document becomes a public document when admitted to probate, available to anyone who asks the court for a copy, while the provisions of a trust may be kept from the public.
It should be noted that assets funneled into a trust through a pour-over will arrangement will be subject to probate costs and delays.